### 10.3 主线程的消息循环 Android的主线程就是[ActivityThread](https://www.androidos.net.cn/android/6.0.1_r16/xref/frameworks/base/core/java/android/app/ActivityThread.java)所在的UI线程，**主线程的入口方法为ActivityThread的main**，**在main方法中系统会通过Looper.prepareMainLooper()来创建主线程的Looper以及MessageQueue，并通过Looper.loop()来开启主线程的消息循环**，这个过程如下所示。 ``` public static void main(String[] args) { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain"); SamplingProfilerIntegration.start(); // CloseGuard defaults to true and can be quite spammy. We // disable it here, but selectively enable it later (via // StrictMode) on debug builds, but using DropBox, not logs. CloseGuard.setEnabled(false); Environment.initForCurrentUser(); // Set the reporter for event logging in libcore EventLogger.setReporter(new EventLoggingReporter()); AndroidKeyStoreProvider.install(); // Make sure TrustedCertificateStore looks in the right place for CA certificates final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId()); TrustedCertificateStore.setDefaultUserDirectory(configDir); Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } // End of event ActivityThreadMain. Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); } ``` 主线程的消息循环开始了以后，**ActivityThread还需要一个Handler来和消息队列进行交互，这个Handler就是ActivityThread.H**，它**内部定义了一组消息类型，主要包含了四大组件的启动和停止等过程**，如下所示。 ``` private class H extends Handler { public static final int LAUNCH_ACTIVITY = 100; public static final int PAUSE_ACTIVITY = 101; public static final int PAUSE_ACTIVITY_FINISHING= 102; public static final int STOP_ACTIVITY_SHOW = 103; public static final int STOP_ACTIVITY_HIDE = 104; public static final int SHOW_WINDOW = 105; public static final int HIDE_WINDOW = 106; public static final int RESUME_ACTIVITY = 107; public static final int SEND_RESULT = 108; public static final int DESTROY_ACTIVITY = 109; public static final int BIND_APPLICATION = 110; public static final int EXIT_APPLICATION = 111; public static final int NEW_INTENT = 112; public static final int RECEIVER = 113; public static final int CREATE_SERVICE = 114; public static final int SERVICE_ARGS = 115; public static final int STOP_SERVICE = 116; ......... } ``` * [ ] **主线程的消息循环模型** **ActivityThread通过它的内部类[ApplicationThread](https://www.androidos.net.cn/android/6.0.1_r16/xref/frameworks/base/core/java/android/app/ActivityThread.java)和AMS进行进程间通信，AMS以进程间通信的方式完成ActivityThread的请求后会回调ApplicationThread中的Binder方法，然后ApplicationThread会向H发送消息，H收到消息后会将ApplicationThread中的逻辑切换到ActivityThread中去执行，即切换到主线程中去执行，这个过程就是主线程的消息循环模型**。  参考文章：[Android中为什么主线程不会因为Looper.loop()里的死循环卡死](https://www.kancloud.cn/alex_wsc/android/1834014)？ [Activity的启动过程？(重点)](https://www.kancloud.cn/alex_wsc/vollery/1826178)